1. Field of the Invention
The present invention relates to a method and an apparatus for accurately measuring residual capacity of a battery employed in an electric vehicle, or more specifically an electric car.
2. Description of the Background Art
So far, a variety of meters, apparatuses and the like for measuring residual electric charge of a dischargeable battery have been proposed. Residual electric charge of a battery is also referred to hereafter simply as battery residual capacity. An example of such equipment is disclosed in Japanese Patent Laid-open No. S50-2130. In addition, addressing environmental problems arising in recent years, there have been rising demands for migration to and development of an electric car as a substitute for the conventional gasoline-powered car. As a result, a residual capacity meter becomes indispensable for accurately measuring the residual electric charge of a battery used in such an electric car.
FIGS. 8 and 9 each show a simplified diagram of an electrical circuit of a meter for measuring battery residual capacity, strictly speaking, for measuring residual electric charge of a battery using a conventional technique known as the battery current integration method.
The flow of current in a process of electrically charging a battery B is shown in FIG. 8. As shown in the figure, a charging current Ic flowing out from an 10 electrical charging unit flows into the battery B. At the same time, the current Ic also flows through a shunt resistor Rs connected in series to the battery B. The shunt resistor Rs allows a residual capacity meter Mi to measure the charge current Ic. Measured values are integrated gradually over the lapse of time to determine the amount of electric charge stored in the battery B. As the battery B enters a fully charged state, the residual capacity meter Mi displays the result of the integration representing the full-charge value as an initial battery residual capacity or, strictly speaking, the amount of electric charge stored initially in the battery B.
On the other hand, FIG. 9 shows a discharge current or a consumed current Io flowing out off the battery B. As shown in the figure, the discharge current Io flows through the shunt resistor Rs and a load. Likewise, the shunt resistor Rs allows the residual capacity meter Mi to measure the discharge current Io as is the case with the charge current Ic. In the case of the discharge current Io, however, measured values are subtracted gradually over the lapse of time from the initial battery residual capacity. A result of the subtraction at a particular time is displayed as a battery residual capacity at that time or, strictly speaking, the amount of residual electric charge currently left in the battery B.
With the above technique, the value of the battery residual capacity expressed in terms of electric charge units called ampere-hours (Ah) can be measured with a relatively high degree of accuracy. In general, however, a battery has a characteristic which shows a variable terminal voltage. The terminal voltage changes depending upon, among other factors, varying conditions due to discharge current consumed by a load and residual electrical charge left in the battery at the time the load is connected. Therefore, the amount of residual capacity measured by the technique described above cannot necessarily be said to represent the residual electric charge actually remaining in the battery. In addition, in the case of an electric car, the travelable distance is proportional to the amount of energy that can be consumed from a battery employed therein. In other words, the distance that can be traveled by the electric car is proportional to the amount of residual capacity in the battery expressed in terms of watt-hours (Wh), an energy unit, instead of ampere-hours (Ah), the electric-charge unit cited above. Therefore, the technique for measuring a battery residual capacity described above is not appropriate for measuring a residual energy in the battery of an electric car indicative of a distance travelable by the electric car.
The reason why the above technique is not suitable for an electric car is explained by referring to FIGS. 10 and 11. These figures each show a discharge characteristic of a battery. FIG. 10 shows the case of a fixed discharge current in which the voltage thus decreases gradually with the lapse of time. The figure shows two equal periods of time (1) and (2). Since the discharge current is constant, electric charges discharged from the battery expressed in terms of ampere-hours (Ah) are the same during both the periods of time (1) and (2,). As shown in FIG. 10, however, the terminal voltage of the battery during the period of time (1) is different from that during the period (2). Accordingly, the energy consumed from the battery expressed in terms of watt-hours (Wh) during the period of time (1) is greater than that in the period (2) due to the fact that the terminal voltage in the former period (1) is higher than that in the latter period (2).
On the other hand, FIG. 11 shows two types of discharge current curves which correspond to discharge currents I1 and I2. Let the magnitudes of the two discharge currents be related by an equation I2=I1*2. Also, assume that a period of time (3) is twice as long as a period of time (4) in length. The Ah value consumed through the discharge current I1 during the period (3) is therefore equal to the Ah value consumed through the current I2 during the period (4). As shown in FIG. 11 however, the terminal voltage of the battery for the discharge current I1, is higher than that for the discharge current I2. Accordingly, the energy consumed from the battery expressed in terms of watt-hours (Wh) through the discharge current I1, is much greater than that consumed through the discharge current I2 due to the fact that the terminal voltage during and the length of the period (3) are greater than the terminal voltage during and the length of the period (4). In other words, with the conventional current integration technique, the discharge currents I1 and I2 result in the same amounts of battery residual charges expressed in terms of ampere-hours. However, the amounts of residual energies expressed in terms of watt-hours actually left in the battery are different. Accordingly, the conventional current integration technique is not suitable for measuring and displaying the amount of energy consumed by a traveling electric car or the amount of residual energy remaining in the battery.
In order to solve the problem described above, a conventional meter Mw for measuring a battery residual capacity shown in FIGS. 12 and 13 is used. The meter Mw adopts a battery power integrating technique. In addition to measuring a current as in the configuration described above, this instrument Mw also measures the terminal voltage of a battery B. To be more specific, during a process of charging the battery B shown in FIG. 12, the residual capacity meter Mw measures a charge current Ic flowing through a shunt resistor Rs as well as a charge voltage Vc of the battery B. The amount of power is then calculated as a product of measured values of the charge current Ic and the charge voltage Vc obtained in this way. Likewise, the calculated power is gradually integrated over the lapse of time. As the battery B enters a fully charged state, the residual capacity meter Mw displays the result of the integration representing the full charge capacity as an initial battery residual capacity. Strictly speaking, the result of integration represents a full charge energy initially stored in the battery B since power is integrated along the time axis.
On the other hand, FIG. 13 shows a discharge current or a consumed current Io flowing out off the battery B. As shown in the figure, the discharge current Io flows through the shunt resistor Rs and a load. Likewise, the residual capacity meter Mw measures the discharge current Io and a discharge voltage Vo to calculate consumed power as is the case with the measurement shown in FIG. 12. In the case of consumed power, however, measured values are subtracted gradually over the lapse of time from the initial battery-residual capacity. A result of the subtraction at a particular time is displayed as a battery residual power at that time or, strictly speaking, a residual energy left in the battery at that time.
In general, an electric car is driven with its battery exhausted after traveling along a fixed distance. Accordingly, the battery is most likely used in a constant-watt discharge mode. With the power integration technique described above, the amount of energy consumed by a traveling electric car or the amount of residual energy is displayed in terms of watt-hours. Therefore, the technique is appropriate for determining the remaining distance along which an electric car can travel.
Nevertheless, the power integration technique described above is effective only if the charge voltage of the battery is equal in magnitude to the discharge voltage thereof. If they are different, a problem will arise. This problem is explained by referring to FIG. 14. Assume that the battery is discharged by a fixed current flow and then, later on, charged with the same current as the discharge current. As shown in the figure, however, the discharge voltage generally tends to be lower than the charge voltage. The difference in voltage is caused by, among other things, a difference in chemical potential between the chemical reactions during charging and discharging processes. The causes of the difference in voltage may also include the effect of an internal resistance of the battery. Thus, a battery residual capacity in terms of Wh cannot be obtained by calculation using an integration result in Wh obtained in the charging process as it is. This is because the Wh integration result obtained in the charging process is calculated with respect to a charge voltage which is higher than the discharge voltage. The result expressed in terms of Wh thus includes a portion that cannot be consumed as an energy required by the electric car to travel. As a result, it becomes difficult to directly apply the power integration technique described above to the calculation of a battery residual capacity even though the technique is regarded as appropriate.